Problem: The scale on a map is 6cm : 9km. If the distance between two cities is 63km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 6cm on the map corresponds to an actual distance of 9km. An actual distance of 63km is the same as 7 $\cdot$ 9km. The distance between the two cities on the map is 7 $\cdot$ 6cm, or 42cm.